Cannot find Mesh: Relax
You need to activate another add-on: Add-ons-&gt;Edit Mesh Tools.
How I understand Relax add-on include in Edit Mesh Tools add-on. This problem was discussed at the bug tracker.
Though the official manual tells us another information:
https://docs.blender.org/manual/en/latest/addons/mesh/edit_mesh_tools.html#
https://docs.blender.org/manual/en/latest/addons/mesh/relax.html
Maybe the developers didn't correct information in the manual.